Question: If $x \bigtriangleup y = x(y-6)$ and $x \boxdot y = 7x+y$, find $5 \boxdot (5 \bigtriangleup -6)$.
Solution: First, find $5 \bigtriangleup -6$ $ 5 \bigtriangleup -6 = 5(-6-6)$ $ \hphantom{5 \bigtriangleup -6} = -60$ Now, find $5 \boxdot -60$ $ 5 \boxdot -60 = (7)(5)-60$ $ \hphantom{5 \boxdot -60} = -25$.